The present invention deals with a new combination of getter materials and the getter devices containing the same. In particular, the present invention relates to a getter combination suitable to the maintenance of vacuum in devices which cannot be heated at temperatures higher than about 200.degree. C.
Getter materials have been found to be practically necessary in all manner of industrial applications that require maintaining a vacuum.
Until a few years ago, the walls designed to confine the vacuum were made of metal or glass in all the devices requiring vacuum for their operation. Evacuated volumes defined by metal walls are present in, for example, "thermos" or "dewars", in the thermally insulated pipes for the conveyance of cryogenic fluids, and in scientific applications such as the particle accelerators. Evacuated volumes defined by glass walls are also present e.g. in the cathode-ray tubes for television screens or computer displays and in lamps. In all these applications, a getter material is introduced into the device in an inactive state before sealing the device. The getter is then activated later, when the device is sealed, by heating it from the outside using, for example, micro- or radio-frequency waves. The activated getter absorbs the last traces of gas still present in the device and carries out the sorption of those gases which, through various mechanisms, enter the evacuated volume during the life of the device itself. The minimum temperatures required by the conventional getter materials for activation are in the order of 350.degree.-400.degree. C., and in some cases even temperatures of about 900.degree. C. can be reached. Getter materials of this type are for example the zirconium- or titanium-based alloys.
However, in recent years, industrial uses of a vacuum have been extended to evacuated devices made, at least in part, of plastic materials. Such materials cannot be heated at temperatures higher than about 200.degree. C. One example is a thermally insulated jacket under vacuum, wherein the plastic materials can be used to form the walls or the filling materials or both. The filling materials ("fillers") are generally fibers, powders of foams and are employed in the jackets to maintain their shape. A typical example of such a jacket is evacuated panels, mainly used in the production of refrigerators. The envelope of these panels is generally made of plastic-metal laminated foils, thermally sealed at their edges through a plastic-to-plastic contact. Metal-to-metal sealing are avoided in order to break the thermal bridge between the two faces of the panel. The plastic materials cannot be heated at temperatures higher than about 200.degree. C. so as to avoid jeopardizing their chemical and mechanical stability. Therefore, the conventional getter materials that require a high activation temperature are inadequate in this type of application. There is a need for getter materials that have a low temperature of activation or, better, that require no thermal activation at all.
International Patent Application WO 94/18876 discloses the combination of an oxide of a noble metal, in particular palladium oxide (PdO), and a moisture sorbing material, such as barium oxide (BaO), for maintaining a vacuum in evacuated jackets of dewars, thermos, etc. However, the palladium oxide disclosed therein reacts with hydrogen and is converted into a finely powdered form of metallic palladium (Pd) having pyrophoric properties. Since this metal can explode and burn, this combination of materials is not recommended for safety reasons.
U.S. Pat. Nos. 5,312,606 and 5,312,607, owned by the assignee of the present application, disclose a family of alloys based on barium and lithium with other elements added such as aluminum or earth-alkaline elements; these alloys are the only known getter materials capable of sorbing practically all gases at room temperature without requiring thermal activation. Specific applications of these materials are described e.g. in the U.S. Pat. No. 5,408,832 and in the International Patent Application WO 96/01966. In particular the preferred alloy is the BaLi.sub.4 alloy. In order to ensure the nitrogen sorption capacity of this alloy, which could become exhausted by the sorption of water vapor, U.S. Pat. No. 5,408,832 discloses the use of BaLi.sub.4 in combination with a moisture sorbing material such as the barium oxide.
This combination of materials shows very good performances as regards the removal of O.sub.2, N.sub.2 and H.sub.2 O, and thus eliminates the principle atmospheric gases from vacuums inside the jackets of the panels. However, the gases which leak into the vacuum of these jackets come mainly from the degassing or out gassing of the materials that form the jackets, in particular the fillers which are generally in the form of powder, foam or wool, and consequently have a large surface area. The main gases being present in the vacuums formed by jackets made of plastic material are CO and CO.sub.2 in case of polymeric filler and H.sub.2 in case of glass wool. The amount of these gases may be significant, especially whenever manufacturing the jacket includes heating steps. For example, in manufacturing refrigerators, vacuum insulating panels are fixed to the walls of the appliances using polymeric foams, generally polyurethanes, obtained by reacting suitable chemical compounds in an in-situ foaming process. This process can achieve temperatures near 100.degree. C. (212.degree. F.) for times of several minutes.
Another major source of the gases which leak into the vacuum of the panels are organic compounds such as hydrocarbons or substituted hydrocarbons in which hydrogen can be replaced partially or completely by halogen atoms. Compounds in which halogen atoms completely replace hydrogen are known as CFCs and have been used for decades in the production of thermal insulating panels for refrigerators. These gases have been recognized as responsible for depleting the ozone layer of the atmosphere, and their production and use have been discontinued. However, thought has been given to recycling old panels containing CFCs by reducing the polymeric foams they contain to powders and then using these powders to produce new panels. Small amounts of CFCs could enter the vacuums of freshly-produced thermal insulating panels in this way.
Partially halogen-substituted hydrocarbons, generally referred to as HCFCs, and hydrocarbons have replaced CFCs in this field, and are used as foaming agents both in the production of panels and in the step of fixing the panels to the refrigerator walls using foams quite similar to those inside the panels. The most important gases in this application are cyclopentane, C.sub.5 H.sub.10, and 1,1-dichloro-1-fluoroethane, Cl.sub.2 FC--CH.sub.3, this latter known in the technique as R141-b. These latter gases can enter the panels through the edges, in the zone where the plastic-metal laminated foils the envelope is made of are sealed through a plastic-to-plastic thermal sealing. This results in an increasing air pressure inside the panel a decreased vacuum and in a general reduction in its thermal insulating properties.
The above described combination, BaO/BaLi.sub.4 can sorb CO, CO.sub.2 and, particularly, H.sub.2, but does so at a relatively low speed; moreover, prior art getter materials are not known to be able to effectively absorb organic compounds.
It is therefore an object of the present invention to provide a combination of getter materials of improved sorption properties for CO, CO.sub.2 and H.sub.2 and capable to absorb organic compounds, which does not require thermal activation and is therefore compatible with devices in which at least one component cannot be heated at temperatures higher than about 200.degree. C.
Another object of the invention is to provide a device for using that combination of getter materials.
According to the present invention, these and other objects are obtained with a combination of getter materials that includes a mixture of an oxide of a transition metal chosen among cobalt oxide, copper oxide or their combinations and metallic palladium containing up to about 2% by weight of metallic palladium, and a moisture sorbing material having a H.sub.2 O vapor pressure lower than 1 Pa at room temperature.
Although various cobalt oxides exist, according to the oxidation number of the metal, the only one which is useful for the invention is the oxide having the empirical formula Co.sub.3 O.sub.4, wherein the cobalt is present at the same time under the oxidation state II and oxidation state III. The following specification and claims use cobalt oxide to mean the compound is as defined herein. Similarly, with copper oxide, the following specification and claims uses CuO compound to include copper in the oxidation state II. Furthermore in the following the abbreviation MO will be used for labeling in general one of the two oxides of the transition metals or a combination thereof, and the abbreviation MO/Pd for indicating the mixture between MO and metallic palladium. The properties of these oxides were already known, for instance by an article by Belousov et al., Ukrainskij Chimiceskij Zurnal, 1986, 52, No. 8, but only for the sorption of hydrogen.
During the preparation of the oxide of the transition metal, a precursor of the metallic palladium is added to the latter in such a quantity to have a final mixture containing up to about 2% by weight of the mixture Mo/Pd. Palladium can be coprecipitated with the oxide of the transition metal by its introduction into the same mother solution in the form of soluble salt, e.g. PdCl.sub.2. As an alternative, palladium may be deposited from a solution onto grains of transition metal oxide being previously formed. The oxide of the transition metal is used in a powdered form with particle size of less than 500 .mu.m and preferably between 1 and 200 .mu.m.
The moisture sorbing material may be chosen from among chemical moisture sorbers. These materials, known in the art, fix the water in an irreversible way through a chemical reaction. Suitable for this application are the chemical dryers having a H.sub.2 O vapor pressure lower than 1 Pa at room temperature, as described in U.S. Pat. No. 5,408,832 of the assignee of this application. For example, the oxides of calcium, strontium, barium and phosphorous or their combinations are considered suitable for achieving the objects of the invention. The use of barium oxide or calcium oxide is particularly preferred. The moisture sorbing material is preferably used in the form of powder having a particular size between about 50 and 500 .mu.m. With a greater particle size an excessive reduction of the surface area of the powder is experienced, whereas with lower particle size there is the risk that, due to the moisture sorption, the powders become excessively compacted, which hinders the passage of gases through the powders themselves. In order to overcome the problem of compaction of humid powders, it is also possible to add to the moisture sorbing material a powder of an inert material, such as alumina, as described in the mentioned International Patent Application WO 96/01966.
The ratio by weight between the materials of the combination of the invention may vary within broad limits, also depending on the type of use that is foreseen and in particular of the gas mixture to be sorbed. However, in general, the ratio by weight between mixture MO/Pd and the moisture sorbing material can vary between about 5:1 and 1:20, and preferably between 1:1 and 1:5.
In one contemplated application, it is foreseen that the vacuum initially present in the jacket can also be degraded by atmospheric gases such as O.sub.2 and N.sub.2. It is possible to also add a barium- and lithium-based alloy among those described in the U.S. Pat. Nos. 5,312,606 and 5,312,607 mentioned before to the combination Mo/Pd with moisture sorber as described above. These patents are incorporated by reference for their details about the preparation and properties of these alloys. The barium- and lithium-based alloy is preferably used in a powdered form with particle size of less than about 500 .mu.m, and preferably less than about 150 .mu.m, in order to increase the surface area. The powder may also be slightly compressed as indicated in the cited Application WO 96/01966. The preferred alloy is that of BaLi.sub.4 composition, mentioned above.
The barium- and lithium-based alloys and the cobalt or copper oxides have a mutual reaction and should therefore be kept separated in order not to cause alterations of the performances of the getter combination.
The ratios by weight between the barium- and lithium-based alloy and the other components of the combination according to the invention can vary within broad ranges. The ratio by weight between mixture MO/Pd and the barium- and lithium-based alloy may generally vary between 10:1 and 1:5 and preferably between 5:1 and 1:2. The ratio by weight between the moisture sorbing material and the barium- and lithium-based alloy may vary approximately between 50:1 and 1:5, preferably between 20:1 and 1:1.
Another aspect of the invention refers to the getter devices containing the combination of materials so far described.